Medical equipment for radiation therapy treats tumorous tissue with high energy radiation. The dose and the placement of the dose must be accurately controlled to insure that the tumor receives sufficient radiation to be destroyed, and that damage to the surrounding and adjacent non-tumorous tissue, referred to as organs at risk (OAR), is minimized.
Radiation therapy typically uses a radiation source that is external to the patient, typically either a radioisotope, such as cobalt-60, or a high energy x-ray source, such as a linear accelerator. The external source produces a collimated beam directed into the patient to the tumor site. However, external-source radiation therapy undesirably irradiates a significant volume of OAR in the path of the radiation beam along with the tumorous tissue. The adverse effect of irradiation of healthy tissue may be reduced, while maintaining a given dose of radiation in the tumorous tissue, by projecting the external radiation beam into the patient at a variety of gantry angles with the beams converging on the tumor site. This reduces the total dose to the healthy tissue during the entire treatment.
The irradiation of healthy tissue also may be reduced by tightly collimating the radiation beam to the general cross section of the tumor taken perpendicular to the axis of the radiation beam. Numerous systems exist for producing such a circumferential collimation, such as systems with multileaf collimators. The multileaf collimator (MLC) may control the width and offset of the radiation beam as a function of gantry angle so that tumorous tissue may be accurately targeted.
Collimation is just one way of shaping the radiation beam. Additionally or alternatively, the radiation beam may be spatially attenuated. Collimators control the outline of the radiation beam; attenuators control the intensity of the radiation beams that are beamed at the tissue. Phrased more technically, collimators block radiation so as to create a generally binary spatial intensity distribution (binary: passed or blocked), while attenuators or beam modulators, typically produce continuous spatial modulation of the beam intensity by selective attenuation.
For example, intensity modulated radiotherapy (IMRT) is aimed at irradiating a target while protecting healthy tissue, especially organs-at-risk (OAR). Intensity modulation is implemented either by multileaf collimators or by attenuating modulators. A desired intensity map is approximated by segmentation: forming a sequence of aperture segments consecutively shaped by the MLC.
Using inverse planning for radiotherapy treatment, the physician prescribes desired target dose and tolerances for sensitive structures, and optimization software explores a multitude of possibilities to determine machine settings so as to closely deliver the prescribed radiation dose. Irradiation is delivered from a discrete set of orientations or from a continuous arc.
In order to modulate radiation beam intensity in a given field, the beam is conceptually partitioned into many small beam segments which are generically called pencil beams or beamlets. Beamlets are indexed by their respective positions in a radiation field and by the orientation of the beam relative to a patient. Dose distribution produced by a beamlet in the patient is calculated and/or measured. Optimizing IMRT amounts to selecting respective intensities of the beamlets, arranged as an intensity map, so as to achieve optimal accumulated dose distribution in the patient. Since separate irradiation of each beamlet is cumbersome, beamlets are grouped into field segments incorporating respectively uniform segment intensities. Successive irradiation of the segments approximates the optimized intensities prescribed by the intensity map. Direct methods optimize the field shape in addition to beamlets intensities, while delivery constraints may be incorporated in the optimization process.
Collimators are configured to define a radiation field by blocking substantially all radiation outside the field aperture. Typically, a desired radiation field is produced by cascaded collimators. Primary, secondary and tertiary collimators are termed according to their respective proximity to the radiation source. A fixed-size stationary primary collimator defines the maximal field size. Secondary collimators are movable and are operable to generally produce rectangular fields of variable size and location. Finer field shaping is further accomplished by tertiary collimators, typically conical (sometimes called cylindrical) collimators of various diameters or multi-leaf collimators. Successive cascaded collimators overlap so as to prevent radiation from leaking between collimators.
A multi-leaf collimator modifies field aperture by adjusting spaces between respective front-ends of opposing leaves. The produced field can be modified during irradiation or between irradiations. Respective rear-ends of MLC leaves overlap the secondary collimator (typically jaws).
While collimators block radiation outside a field, beam intensity can be modulated by a physical modulator covering the whole field. Such a modulator incorporates spatially variable attenuating properties tailored to a specific intensity map. Simple modulators, e.g., a wedge, do not generally provide on their own IMRT with sufficient quality.